First thing to do is figure out what the tank is made of. Likely it is ABS or HDPE. Since you have a spin welded fitting, it is likely a thermoplastic like HDPE, and not very much sticks to that.
There are a few threads on these forums that deal with tank repairs, try searching for "tank repair" and you should get some hits. I recall one respondent saying they found a glue that would adhere to HDPE.
If you have clear access to the broken fitting, you might try just cutting the broken piece off flush with the tank, and spin-welding a new one on top of it.